Critical properties of nanoporous low dielectric constant films revealed by Brillouin light scattering and surface acoustic wave spectroscopy

Thin porous films with nanometer pore sizes are the subject of intense interest, primarily because of their reduced dielectric constant $\mathrm{k.}$ The lack of useful characterization tools and the reduction in film mechanical properties with increasing porosity have severely hindered their development and application. We show that both Brillouin light scattering and surface acoustic wave spectroscopy allow one to measure density, porosity and stiffness properties of nanoporous methylsilsesquioxane films of low-$k$ value. Excellent correlations are observed among independent measurements of density, porosity and the Young’s modulus which show that the results obtained are reliable and reveal properties of the films which are difficult or impossible to obtain using other techniques.